1. Field of the Invention
The present invention is generally related to communication systems. More particularly, the present invention is related to cable modem systems and methods for ranging cable modems.
2. Background Art
In conventional cable modem systems, a hybrid fiber-coaxial (HFC) network provides a point-to-multipoint topology for supporting data communication between a cable modem termination system (CMTS) at the cable headend and multiple cable modems (CM) at the customer premises. In such systems, information is broadcast downstream from the CMTS to the cable modems as a continuous transmitted signal in accordance with a time division multiplexing (TDM) technique. In contrast, information is transmitted upstream from each of the cable modems to the CMTS as short burst signals in accordance with a time division multiple access (TDMA) technique. The upstream transmission of data from the cable modems is managed by the CMTS, which allots to each cable modem specific slots of time within which to transfer data.
Conventional cable modem systems utilize DOCSIS-compliant equipment and protocols to carry out the transfer of data packets between multiple cable modems and a CMTS. The term DOCSIS (Data Over Cable System Interface Specification) refers to a group of specifications published by CableLabs that define industry standards for cable headend and cable modem equipment. The most recent version of DOCSIS is DOCSIS 2.0. In part, DOCSIS sets forth requirements and objectives for various aspects of cable modem systems including ranging, operations support systems, management, data interfaces, as well as network layer, data link layer, and physical layer transport for data over cable systems.
Ranging is the process used to adjust transmit levels and time offsets of individual cable modems in order to make sure the data coming from the different modems line up in the right time slots and are received at the same power level at the CMTS. Ensuring that data arrives at the same power level is essential for detecting collisions. If two Cable Modems transmit at the same time, but one is weaker than the other one, the CMTS will only hear the strong signal and assume everything is okay. However, if the two signals have the same strength, the signal will garble and the CMTS will know a collision occurred. Carrier frequency adjustments are also made during the ranging process. Adjustments to the carrier frequency are made to ensure that the CM and CMTS are exchanging transmissions at an agreed upon frequency.
Pre-equalization is a process for reducing distortion over the transmission path between the CM and the CMTS and is a part of the above described ranging process. Pre-equalization is achieved using pre-equalization coefficients. The DOCSIS specification provides guidelines for how the pre-equalization coefficients are utilized; however, it does not stipulate as to how the coefficient values are derived.
It has been observed that cable modem systems have not been operated according to their greatest potential. Accordingly, what is desired is a system and method for optimizing the ranging process. In this way robust operation between the multiple cable modems and the cable modem termination system in the HFC network can be achieved.